Line a small loaf pan or an 8-inch square pan with parchment paper, leaving a little overhang for lifting the candy out later. Lightly butter the parchment.

Melt the unsalted butter in a heavy-bottomed saucepan over medium heat. Add the allulose and heavy whipping cream, stirring until the mixture is evenly combined.

Bring the mixture to a steady simmer, then cook while stirring frequently for approximately 15–25 minutes. Continue until the caramel thickens, looks glossy, and reaches about 240–250°F (115–121°C) on a candy thermometer.

Remove the saucepan from the heat and carefully stir in the vanilla extract and fine sea salt. The mixture will be extremely hot and may bubble as the flavorings are added.

Pour the hot caramel into the prepared pan and spread it gently into an even layer. If desired, sprinkle a small amount of flaky sea salt over the surface immediately.

Allow the pan to cool at room temperature, then refrigerate for 1–2 hours or overnight for firmer pieces. Lift the chilled caramel from the pan, cut it into 20–24 small squares, and wrap each piece in parchment or waxed paper.